Although the pulmonary inflammatory disease following aerobic gram negative bacterial infections is severe and sometimes lethal, the nature of the specific host cells that initiate the inflammatory process is unclear. Mast cells, which are known primarily for their role in IgE- mediated allergic reactions, are suited to initiate and regulate inflammation against pulmonary pathogens because of (i) their abundance at the host-environment interface in the lung (20,000/mm3) and (ii) their ability to spontaneously release various multifunctional mediators of inflammation. However, there is currently little recognition of mast cell participation in infectious diseases. One reason for this void in mast cell biology is the lack of knowledge of how mast cells recognize and respond to infectious agents. We have discovered that mast cells can interact with the pulmonary pathogen Klebsiella pneumoniae (Kp) through a mannose-binding protein (FimH) borne on the tips of surface organelles of these bacteria. This interaction triggers a cascade of activity in the mast cell including tyrosine phosphorylation of several prominent proteins, oxidative metabolism and release of potent inflammatory mediators. We have also shown that the presence of IgE antibodies bound on the surface of mast cells (a common condition in allergic subjects) appear to promote bacteria-mast cell interactions raising the possibility that this class of antibody could induce mast cell hyperresponsiveness to bacteria. In this proposal, we plan to extend some of these intriguing observations. Therefore, our aims are (1) to isolate and characterize the cognate mast cell membrane molecule(s) that bind Kp FimH, (2) to localize and characterize mast cell proteins that are tyrosine phosphorylated upon activation by Kp FimH expressing (FimH+) bacteria, (3) to examine the modulatory effect of IgE on the adherence of Kp FimH+ bacteria to mast cells and on the subsequent intracellular signaling events preceding mast cell release of mediators and (4) to investigate the role of mast cells and two of their prominent stored mediators (TNFalpha and histamine) in modulating bacterial clearance and pulmonary inflammation in genetically mast cell deficient mice and their normal littermate controls. These studies should serve as a paradigm of mast cell-bacteria interactions and provide valuable information on (i) the identity and structure of mast cell receptor(s) for bacteria (ii) tyrosine phosphorylation in mast cells triggered by bacterial adhesion proteins (iii) the role of IgE in augmenting bacterial inflammation and (iv) the role of mast cells and their mediators in regulating inflammatory response to respiratory infection in vivo. Finally, these studies should provide vital clues to developing new and broadly applicable strategies to augment defense against pulmonary infections.